1.Field of the Invention
The present invention relates to a running control device for generating a yaw moment to stabilize a running condition of a vehicle such as an automobile at least when the running condition is deteriorated, and more specifically, to such a device for controlling a steering of a vehicle, as well as its behavior stability.
2.Description of Prior Art
A running behavior of a vehicle can be stabilized through generating a yaw moment around the centroid of the vehicle. In well-known Vehicle Stability Control (VSC), the tire force distribution in a vehicle is controlled to generate a yaw moment in a direction opposite to oversteering, understeering and other undesirable yaw directional tendencies of the vehicle (a counter yaw moment), assisting a driver's steering operation for suppressing such undesirable tendencies.
Further, in a vehicle equipped with a steering system enabling the steering of wheels independently of a driver's steering operation, a yaw moment to assist the driver for suppressing running behavioral deterioration may also be generated by an automatic steering of wheels. For example, in Japanese Patent Laid-Open Publication No. 5-105055 and Japanese Patent No. 2540742, a steering angle of wheels is controlled to generate a counter yaw moment against a braking force imbalance between the left and right wheels on a vehicle, and thereby ensuring its straight line stability. Japanese Patent Laid-Open Publication No. 6-135345 proposes controlling an auxiliary steering system in a four wheel steering vehicle using proportional, differential and integral terms of the deviation of an actual yaw rate from its target value, improving the responsiveness of the auxiliary steering system against a lateral sliding of a vehicle body owing to an abrupt steering in the main steering system, crosswind disturbances, etc.
Oversteering and/or understeering tendencies and other yaw behavioral tendencies, owing to tire force saturation on front and/or rear wheels of the vehicle, tire force imbalance between left and right wheels, etc., primarily appear in a yaw rate of the vehicle. Thus, in conventional devices for stabilizing a running behavior of a vehicle, such as VSC devices, typically, a counter yaw moment is generated to reduce the deviation of an actual yaw rate from its target value, i.e. an ideal yaw rate, which can be determined based upon a theoretical motional equation using a steering angle of wheels or a handle, a vehicle speed, etc. as parameters. The adjusting of the actual yaw rate to its target value will suppress further yaw behavioral deterioration, thereby preventing spinning and/or drifting-out, unintentional turning, yaw directional oscillation of a vehicle.
The above-mentioned control strategy of adjusting a yaw rate in the conventional devices, however, could not fully correct the deviation of a yaw angle of a vehicle body from the yaw angle consistent with the traveling direction intended by a driver. In the strategy of adjusting a yaw rate, the generation of a counter yaw moment would be ended when an actual yaw rate coincides with its target value even if the direction of the vehicle body, namely, the yaw angle of the vehicle, remains inconsistent with the direction of a traveling course intended by a driver. In this regard, for a normal vehicle, maneuvered by the handling of a driver, a target yaw angle could not be determined with a control device theoretically like the target yaw rate. Since the target yaw angle is inherently to be determined by a driver's intention and the relation between a steering angle (an input of the driver's intention), and an actual traveling direction of a vehicle widely varies with the running condition of a vehicle, the driver has to adjust the yaw angle through his handling while confirming the actual traveling course of his vehicle.
In the purpose of vehicle stability control, i.e. assisting a driver's steering operation, however, it will be preferable to correct at least a tendency of a yaw angle deviating from a driver's intention. Accordingly, conventional vehicle stability and/or steering control devices may be improved to operate more appropriately while taking into account the yaw angle deviation owing to the yaw rate deviation.